In the case of a wheel cap of a car, for example, from a viewpoint of a reduction in weight and so forth, a plastic wheel cap is now on the market in place of the conventional metallic wheel. As resins for production of such plastic wheel caps, modified polyphenylene oxide (modified PPO) and polyamide (PA) are mainly used.
These resins, however, are expensive and it is thus desirable to replace them with other resins. Further, there is a demand to reduce the weight of the wheel cap.
The present inventors made extensive investigation of polypropylene (PP) capable of meeting the above requirements. More particularly, they made a study of a fiber reinforced resin, i.e., a FRPP resin composition, because PP alone is not sufficiently strong (such as with regard to stiffness and impact resistance).
In view of adhesion properties between PP polymer and reinforcing fibers, a mixed polymer of PP and a carboxyl group-containing PP (hereinafter referred to as "modified PP") was used as the polymer, and silane treated fibers were used as the reinforcing fibers.
In order to produce a gray colored wheel cap, a coloring component comprising two component pigments of zinc sulfide and carbon black with a general dispersing agent (higher fatty acid metal salt) added thereto was compounded to the above FRPP resin composition to prepare a molding material.
It has been discovered, however, that by injection molding of the above molding material (FRPP resin composition) is difficult to obtain a molding having sufficient strength (both stiffness and impact resistance) to be used as a wheel cap (see Comparative Example 1).